1. Field of the Invention
This invention relates to a method and a device for sharpening spiral and twist drill bits on the flat surface of a grinding wheel of a bench grinder or on the flat surface of a sharpening stone.
2. Prior Art
The number of commercially available drill bit grinders, or sharpeners, both powered and manual, are few in number. What is available is either too expensive for home use or is too complicated to consistently achieve the same results. Consequently, a large number of blunt and worn drill bits are being thrown away. Some powered drill bit grinders produce satisfactory results but are too expensive or too complicated to use and require frequent changing of the grinding wheel since no redressing of the grinding surface is provided during grinding. Others do not produce satisfactory results, due primarily to wrong assumptions made as to how a drill bit should be reground.
A review of some of the U.S. patents granted on drill bit grinders or sharpeners indicates three basic types: first, the completely automated type where the grinding wheel and the grinding apparatus are incorporated into one unit such as that disclosed in U.S. Pat. Nos. 4,031,672 and 4,001,975; second, a sharpener device employing an electric drill such as U.S. Pat. No. 4,744,178; and third, a manual type of sharpener which includes a means for supporting the drill bit during grinding either on a grinding wheel or a flat surface sharpening stone such as disclosed in U.S. Pat. Nos. 3,961,450; 4,164,100 and 4,566,227. A review of the grinding devices described in these U.S. patents indicates certain inadequacies that can be improved upon.
U.S. Pat. No. 3,961,450 describes a hand-held, portable sharpening device with a holder for the twist drill and an optical indicating device for sharpening using a hand-held, flat-surfaced sharpening stone. Once it is positioned in the holder, a twist drill is ready for sharpening of each cutting edge without further adjustments by moving the sharpening stone along the end faces of a pair of clamping pieces. Although simple in construction, this invention works well with small bits up to 1/8 inch in diameter but is too labor intensive for larger twist drills which require excessive amount of time to sharpen. This invention requires a lot of skill to provide satisfactory results since no means is provided for positioning the drill with respect to the grinding surface at the correct angle for obtaining the correct lip clearance angle.
U.S. Pat. No. 4,744,178 discloses a grinding device including a grinding wheel which is rotated by an electric drill, a guide to direct the drill bit at a prescribed angle to the grinding wheel with the guide being rotatable about a pivot axis to relieve the trailing edge. Due to the high speed of the hand-held electric drill and the small size of the grinding wheel that can be attached to the guide fixture, it is very difficult to control the rate of grinding and, consequently, to achieve equal cutting edges and correct clearance angles. The device is also not very suitable for grinding larger drill bits which require a larger grinding surface area for accuracy.
U.S. Pat. No. 4,031,672 describes an automatic sharpener where the grinding wheel is incorporated into the sharpener. A rotatable chuck holds the drill bit. Apparatus is provided for mounting the chuck relative to the grinding wheel so that a drill bit can be moved in a prescribed manner across the grinding periphery of the grinding wheel. A means for moving the drill bit towards the grinding wheel is provided A gauge is also provided to gauge the distance between the chuck and the surface of a grinding wheel. Since the chuck of this invention blocks the drill bit during grinding as the twist drill is advanced toward the surface of the grinding wheel, the grinding procedure has to be repeated a large number of times before a correct lip length for both cutting edges and the correct lip relief angle can be achieved.
U.S. Pat. No. 4,658,549 discloses a drill bit sharpening device which includes a drill-holding jig which includes a frame member. The frame member carries a pivot arm which is placed in a pivot arm retaining means portion of an adjustable support. A drill bit is held in the jig so that it intersects a grinding face of a grindstone at a predetermined lateral angle.
In summary, none of the patents reviewed, except for the expensive automated type, provides an accurate means of sharpening twist drill bits of all sizes, or provides adequate mathematical support for achieving the correct lip relief angle during grinding The drill bit sharpener described herein is designed to overcome these disadvantages associated with previous sharpeners by providing an accurate means for sharpening drill bits of various sizes with minimum amounts of setup time and grinding time. The required accuracy is derived from a mathematical formula that describes the inter-relationship between critical drill bit design parameters such as point angle and lip relief angle.